Transistors, memory cells and semiconductor constructions

ABSTRACT

Some embodiments include a semiconductor construction having a gate extending into a semiconductor base. Conductively-doped source and drain regions are within the base adjacent the gate. A gate dielectric has a first segment between the source region and the gate, a second segment between the drain region and the gate, and a third segment between the first and second segments. At least a portion of the gate dielectric comprises ferroelectric material. In some embodiments the ferroelectric material is within each of the first, second and third segments. In some embodiments, the ferroelectric material is within the first segment or the third segment. In some embodiments, a transistor has a gate, a source region and a drain region; and has a channel region between the source and drain regions. The transistor has a gate dielectric which contains ferroelectric material between the source region and the gate.

RELATED PATENT DATA

This patent resulted from a from a continuation of U.S. patentapplication Ser. No. 14/991,792, which was filed Jan. 8, 2016, whichissued as U.S. Pat. No. 9,590,066 and which is hereby incorporatedherein by reference; which resulted from a continuation of U.S. patentapplication Ser. No. 14/331,026, which was filed Jul. 14, 2014, whichissued as U.S. Pat. No. 9,263,672, and which is hereby incorporatedherein by reference; which resulted from a divisional of U.S. patentapplication Ser. No. 13/682,190, which was filed Nov. 20, 2012, whichissued as U.S. Pat. No. 8,796,751, and which is hereby incorporatedherein by reference.

TECHNICAL FIELD

Transistors, memory cells and semiconductor constructions.

BACKGROUND

Memory is one type of integrated circuitry, and is used in computersystems for storing data. Integrated memory is usually fabricated in oneor more arrays of individual memory cells. The memory cells may bevolatile, semi-volatile, or nonvolatile. Nonvolatile memory cells canstore data for extended periods of time, and in some instances can storedata in the absence of power. Volatile memory dissipates and istherefore refreshed/rewritten to maintain data storage.

The memory cells are configured to retain or store information in atleast two different selectable states. In a binary system, the statesare considered as either a “0” or a “1”. In other systems, at least someindividual memory cells may be configured to store more than twoselectable states of information.

Dynamic random access memory (DRAM) is one type of memory, and isutilized in numerous electronic systems. A DRAM cell may comprise atransistor in combination with a charge storage device (for instance, acapacitor). DRAM has an advantage of having rapid read/write; but hasdisadvantages of being highly volatile (often requiring refresh ofseveral hundreds of times per second) and of being erased in the eventof power loss.

It is desired to develop improved memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a portion of asemiconductor construction illustrating an example embodiment transistorincorporated into an example embodiment memory cell.

FIG. 2 diagrammatically illustrates the memory cell of FIG. 1 in twodifferent example memory states.

FIGS. 3-7 diagrammatically illustrate example embodiment transistorsincorporated into example embodiment memory cells.

FIG. 8 illustrates another embodiment memory cell comprising the exampleembodiment transistor of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Some embodiments include transistors which comprise ferroelectricmaterial incorporated into gate dielectric. In some embodiments, suchtransistors may be incorporated into memory cells. Example embodimentsare described with reference to FIGS. 1-8.

Referring to FIG. 1, an example embodiment memory cell 40 is illustratedas part of a semiconductor construction 10.

The construction 10 includes a base 12. The base 12 may comprisesemiconductor material, and in some embodiments may comprise, consistessentially of, or consist of monocrystalline silicon. In someembodiments, base 12 may be considered to comprise a semiconductorsubstrate. The term “semiconductor substrate” means any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials), and semiconductive materiallayers (either alone or in assemblies comprising other materials). Theterm “substrate” refers to any supporting structure, including, but notlimited to, the semiconductor substrates described above. In someembodiments, base 12 may correspond to a semiconductor substratecontaining one or more materials associated with integrated circuitfabrication. Some of the materials may be under the shown region of base12 and/or may be laterally adjacent the shown region of base 12; and maycorrespond to, for example, one or more of refractory metal materials,barrier materials, diffusion materials, insulator materials, etc.

A transistor gate 14 extends into base 12. The transistor gate comprisesgate material 16. Such gate material may be any suitable composition orcombination of compositions; and in some embodiments may comprise,consist essentially of, or consist of one or more of various metals (forexample, tungsten, titanium, etc.), metal-containing compositions (forinstance, metal nitride, metal carbide, metal silicide, etc.), andconductively-doped semiconductor materials (for instance,conductively-doped silicon, conductively-doped germanium, etc.). In someexample embodiments, the gate material 16 may comprise, consistessentially of, or consist of one or more of titanium nitride, titaniumaluminum nitride, tungsten nitride, copper and tantalum nitride.

Gate dielectric 18 is between gate material 14 and base 12. The gatedielectric is configured as an upwardly-opening container 24 along thecross-section of FIG. 1, and the gate 14 is within such container. Thegate dielectric comprises two separate materials 20 and 22 in theembodiment of FIG. 1, which may be referred to as a first material and asecond material, respectively. The first material 20 forms an outerboundary of the container 24, and is directly against the semiconductorbase 12. The second material 22 is between the first material 20 and thegate 14. In some embodiments, the first material 20 is anon-ferroelectric material, and the second material is a ferroelectricmaterial. In such embodiments, the first material 20 may comprise,consist essentially of, or consist of one or both of silicon dioxide andsilicon nitride; and the second material 22 may comprise, consistessentially of, or consist of one or more of yttrium-doped zirconiumoxide, yttrium-doped hafnium oxide, magnesium-doped zirconium oxide,magnesium-doped hafnium oxide, silicon-doped hafnium oxide,silicon-doped zirconium oxide and barium-doped titanium oxide.Accordingly, in some embodiments the first material 20 may comprise oneor more of silicon, nitrogen and oxygen; and the second material 22 maycomprise one or more of Hf, Zr, Si, O, Y, Ba, Mg and Ti.

In some embodiments, the ferroelectric material 22 may have a thicknesswithin a range of from about 10 angstroms to about 200 angstroms, andthe non-ferroelectric material 20 may have a thickness within a range offrom about 10 angstroms to about 20 angstroms.

Construction 10 comprises a conductively-doped source region 26extending into base 12, and a conductively-doped drain region 28extending into the base. Lower boundaries of the source and drainregions are diagrammatically illustrated with dashed lines. The sourceand drain regions are both adjacent to gate 14, and are spaced from thegate by the gate dielectric 18. The source and drain regions are spacedfrom one another by a channel region 30 that extends under the gate 14.

In some embodiments, the source region 26 may be referred to as a firstregion of the base adjacent to the gate 14, and the drain region 28 maybe referred to as a second region of the base adjacent to the gate. Suchfirst and second regions of the base are spaced from one another by anintervening region of the base comprising the channel region 30.

The gate dielectric 18 may be considered to comprise a first segment 23between the source region 26 and the gate 14, a second segment 25between the drain region 28 and the gate 14, and a third segment 27between the first and second segments. In some embodiments, the segment23 may be considered to correspond to a first substantially vertical legof container 24, the segment 25 may be considered to correspond to asecond substantially vertical leg of the container, and the segment 27may be considered to comprise a bottom of the container.

In the shown embodiment, all of the first, second and third segments(23, 25 and 27) of gate dielectric 18 comprise ferroelectric material22. In other embodiments (some of which are discussed below withreference to FIGS. 4-6), the ferroelectric material 22 may be omittedfrom one or more of such segments.

In some embodiments, the non-ferroelectric material 20 provides abarrier between ferroelectric material 22 and base 12 to avoid undesireddiffusion of constituents between the ferroelectric material and thebase and/or to avoid undesired reaction or other interaction between theferroelectric material and the base. In such embodiments, thenon-ferroelectric material 20 may be provided entirely along an outeredge of the gate dielectric (as shown) to form a boundary of thecontainer 24 against the semiconductor base 12 (with source and drainregions 26 and 28 being considered to be part of the base). In someembodiments, diffusion and/or other interactions are not problematicrelative to the ferroelectric material 22 even in the absence of atleast some of the non-ferroelectric material, and accordingly some orall the non-ferroelectric material 20 may be omitted from one or more ofthe segments 23, 25 and 27.

In the shown embodiment, the non-ferroelectric material 20 is asubstantially consistent thickness along an entirety of container 24. Inother embodiments (one of which is discussed below with reference toFIG. 7), the non-ferroelectric material 20 may have a differentthickness along one region of container 24 as compared to anotherregion.

In the shown embodiment, source region 26 is electrically coupled tocircuitry 32, drain region 28 is electrically coupled to circuitry 34,and gate 14 is electrically coupled to circuitry 36. A transistor 38comprises the gate 14 together with the source/drain regions 26 and 28,and such transistor is incorporated into an integrated circuit throughcircuitry 32, 34 and 36.

Although the embodiment of FIG. 1 utilizes transistor 38 as part of amemory cell 40, in other embodiments the transistor 38 may be utilizedin other applications. For instance, transistor 38 may be utilized inlogic or other circuitry in place of a conventional transistor.

The ferroelectric material 22 of gate dielectric 18 may be polarizedinto either of two stable orientations, which may enable two selectablestates of memory cell 40. Example memory states are shown in FIG. 2,with the memory states being labeled as “MEMORY STATE 1” and “MEMORYSTATE 2”. The illustrated memory cell of FIG. 2 has n-type doped sourceand drain regions 26 and 28, and a p-type doped channel region. In otherembodiments, the source and drain regions may be p-type doped and thechannel region may be n-type doped.

MEMORY STATE 1 and MEMORY STATE 2 differ from one another relative tothe orientation of charge within ferroelectric material 22. Such chargeorientation is diagrammatically illustrated with “+” and “−” in thediagrammatic illustrations of FIG. 2. Specifically, the memory states ofFIG. 2 are shown to differ from one another relative to chargepolarization within ferroelectric material 22. A double-headed arrow 41is provided in FIG. 2 to diagrammatically illustrate that the memorycell 40 may be reversibly transitioned between the shown memory states.

In the shown embodiment, the polarization change within ferroelectricmaterial 22 specifically occurs within the region 23 between gate 14 andsource region 26 (the polarization change may also occur in otherregions, such as adjacent the channel in some embodiments; or may occuronly in the region 23 as shown in FIG. 2). The MEMORY STATE 1 comprisesa “+” component of the polarized ferroelectric material along the n-typedoped source region 26, and the MEMORY STATE 2 comprises a “−” componentof the polarized ferroelectric material along the n-type doped sourceregion 26. The “−” component of the ferroelectric material is shown toinduce a depletion region 42 within the n-type doped source region 26 (aboundary of the depletion region is diagrammatically illustrated withthe dashed line 43). In the illustrated embodiment, the depletion region42 is deep within the source region 26, and specifically is along aportion of the source region that interfaces with channel region 30. Thetransistor 38 may have an increased effective channel length relative toan analogous transistor lacking the depletion region, which may reduceshort channel effects and thereby improve scalability of the memory cellfor higher levels of integration.

In the shown embodiment, the non-ferroelectric material 20 is betweenferroelectric material 22 and source region 26, and accordingly thedepletion region 42 is spaced from the ferroelectric material 22 by asegment of non-ferroelectric material 20. In other embodiments, thenon-ferroelectric material 20 may be omitted, and the depletion region42 may directly contact the ferroelectric material 22.

The memory cell 40 of FIG. 2 may have advantages of being substantiallynonvolatile, and of retaining stored information in the absence ofpower.

The memory cell 40 may be programmed with any suitable operation, and insome example embodiments may be programmed utilizing voltagedifferentials between gate 14 and source 26 of less than or equal toabout 10 volts; in some example embodiments utilizing voltagedifferentials of less than or equal to about 5 volts; and in someexample embodiments utilizing voltage differentials of from about 0.5volts to about 5 volts.

The dopant concentrations utilized within source region 26 and drainregion 28 may be any suitable dopant concentrations. In someembodiments, the drain region may be more heavily doped than at leastsome of the source region; and in some embodiments the entirety of thedrain region may be more heavily doped than any portion of the sourceregion. In some embodiments, relatively heavy doping of the drain regionalleviates influence of ferroelectric polarization on operation of thedrain side of transistor 38, while relatively light doping of at leastsome of the source region enables the influence of the ferroelectricpolarization on the source side of the transistor to be enhancedrelative to the influence that would occur with heavier doping of thesource region. The terms “relatively heavy doping” and “relatively lightdoping” are utilized with reference to one another, and thus the term“relatively heavy doping” means doping heavier than the doping indicatedby the term “relatively light doping”.

In some embodiments the drain region 28 may be n-type doped, and some orall of the drain region may comprise a dopant concentration of at leastabout 1×10²⁰ atoms/centimeter³; such as, for example, a dopantconcentration within a range of from about 1×10¹⁸ atoms/centimeter³ toabout 1×10²⁰ atoms/centimeter³. In some embodiments the source region 26may be n-type doped, and at least some of the source region may comprisea dopant concentration of less than about 1×10²⁰ atoms/centimeter³; suchas, for example, a dopant concentration within a range of from about1×10¹⁶ atoms/centimeter³ to about 1×10^(19.5) atoms/centimeter³.

In some embodiments, the source region 26 may comprise a gradient ofdopant concentration, with dopant concentration being lighter at deeperlocations of the source region as compared to shallower locations of thesource region. FIG. 3 shows a construction 10 a illustrating an exampleembodiment memory cell 40 a having decreasing dopant concentration withincreasing depth in the source region, (the dopant concentration isillustrated as [DOPANT]). The construction of FIG. 3 advantageously maycomprise the lighter dopant concentration within the source region at alocation where the depletion region 42 forms during programming of amemory state analogous to the MEMORY STATE 2 of FIG. 2.

The example embodiment memory cell 40 shown in FIG. 1 comprises bothferroelectric material 22 and non-ferroelectric material 20 within allof the segments 23, 25 and 27 of dielectric material 18. FIG. 4 shows analternative example embodiment memory cell 40 b having ferroelectricmaterial 22 only within segment 23.

The memory cell 40 b is part of a construction 10 b, and comprises atransistor 38 b containing gate dielectric 18 b. The gate dielectric 18b comprises the non-ferroelectric material 20 between ferroelectricmaterial 22 and source region 26, and comprises additionalnon-ferroelectric material 50 throughout the segments 25 and 27 (i.e.,the segments along drain region 28 and channel region 30). Thenon-ferroelectric material 50 may comprise any suitable composition orcombination of compositions. In some embodiments, the non-ferroelectricmaterial 50 may comprise a same composition as non-ferroelectricmaterial 20, and in other embodiments may comprise a differentcomposition than non-ferroelectric material 20. In some embodiments,non-ferroelectric material 50 may comprise, consist essentially of, orconsist of one or both of second dioxide and second nitride.

The memory cell 40 b of FIG. 4, like the above-discussed embodiment ofFIG. 1, comprises non-ferroelectric material entirely along an interfaceof the source region 26 and the gate dielectric, and entirely along aninterface of the drain region 28 and the gate dielectric. FIG. 5 shows amemory cell analogous to that of FIG. 4, but in which an interface ofthe gate dielectric with the source region comprises ferroelectricmaterial. Specifically, FIG. 5 shows a construction 10 c comprising amemory cell 40 c having a transistor 38 c with gate dielectric 18 c. Thegate dielectric 18 c comprises ferroelectric material 22 andnon-ferroelectric material 50. The ferroelectric material 22 directlycontacts both the source region 26 and the gate 14.

In the shown embodiment, a portion of the segment of the gate dielectricbetween the source region and the transistor gate (i.e., a portion ofthe segment 23 of the gate dielectric) consists of ferroelectricmaterial, and the remainder of the gate dielectric (i.e., the remaindersegment 23, together with segments 25 and 27) consists ofnon-ferroelectric material. In the shown embodiment, only a portion ofan interface between the gate dielectric 18 c and the source region 26consists of ferroelectric material 22. In other embodiments, an entiretyof the interface between the gate dielectric and the source region mayconsist of the ferroelectric material.

FIG. 6 shows a construction 10 d illustrating another example embodimentmemory cell 40 d. The memory cell comprises a transistor 38 d havinggate dielectric 18 d. The gate dielectric comprises non-ferroelectricmaterial 50 throughout the entirety of the segment between the sourceregion 26 and the gate 14 (i.e., the segment 23), and throughout theentirety of the segment between the drain region 28 and the gate 14(i.e., the segment 25). The gate dielectric further comprisesferroelectric material 22 within at least some of the segment along thechannel region 30 (i.e., the segment 27). Such may enable selectivecoupling of the ferroelectric material with the channel region,exclusive of coupling between the ferroelectric material and the sourceregion and/or drain region, which may enable operational characteristicsof the memory cell to be tailored for particular applications. Further,if transistor 38 d is utilized in place of a conventional transistor inan integrated circuit application other than as a part of a memory cell,the selective coupling to the channel region may enable operationalaspects of such transistor to be tailored for specific applications.

The embodiment of FIG. 6 shows the non-ferroelectric material 20provided between ferroelectric material 22 and base 12. In otherembodiments, the non-ferroelectric material 20 may be omitted so thatferroelectric material 22 directly contacts base 12.

Another example embodiment memory cell 40 e is shown in FIG. 7 as partof a construction 10 e comprising a transistor 38 e with gate dielectric18 e. The memory cell 40 e of FIG. 7 is similar to the memory cell 40 ofFIG. 1, in that the memory cell 40 e comprises both thenon-ferroelectric material 20 and the ferroelectric material 22 withinall of the segments 23, 25 and 27 of the gate dielectric. However,unlike the embodiment of FIG. 1, that of FIG. 7 has thenon-ferroelectric material 20 thicker within the segment 27 (i.e. alongthe bottom of the container 24 defined by the gate dielectric) thanwithin the segments 23 and 25 (i.e., along the substantially verticallegs of the container 24 defined by the gate dielectric). Such canalleviate or eliminate coupling between the ferroelectric material 22and the channel 30, which may be desired in some embodiments. In someembodiments, the non-ferroelectric material 20 may have a thicknesswithin segments 23 and 25 within a range of from about 10 angstroms toabout 20 angstroms, and may have a thickness along the bottom ofcontainer 24 within a range of from about 25 angstroms to about 50angstroms.

In some embodiments, the memory cells described above may compriseDRAM-type cells. For instance, the circuitry 34 may correspond to acharge-storage device (such as, for example, a capacitor), the circuitry32 may include an access/sense line (such as, for example, a bitline),and the circuitry 36 may include a wordline that extends in and out ofthe page relative to the cross-sections of FIGS. 1-7. FIG. 8 shows aconstruction 10 f comprising the transistor 38 of FIG. 1 incorporatedinto a DRAM-type memory cell 80.

The DRAM-type cell of FIG. 8 may be, in a sense, considered to includeboth a volatile memory storage component (the capacitor 70, with suchcomponent storing data by utilizing different charge states of thecapacitor as different memory states) and a nonvolatile memory storagecomponent (the transistor 38, with such component storing data byutilizing different polarization orientations of ferroelectric material22 as different memory states, as discussed above with reference to FIG.2).

The volatile memory storage component may have rapid read/writecharacteristics analogous to those of a conventional DRAM, and thenonvolatile memory storage component may enable the cell to havecapabilities beyond those of conventional DRAM. For instance, in someembodiments the cell may be configured so that the nonvolatile memorystorage component backs up information from the volatile memory storagecomponent so that the information is stable in the event of powerfailure. As another example, in some embodiments the cell may beconfigured so that the nonvolatile memory storage component is utilizedfor operations separate from those conducted by the volatile memorystorage component and/or for operations that modify or overlap those ofthe volatile memory storage component. Such may enable a DRAM arraycomprising memory cells 80 of the type shown in FIG. 8 to performoperations that would otherwise comprise both logic and memory aspectsof conventional integrated circuitry, which may enable a DRAM arraycomprising memory cells 40 of the type shown in FIG. 8 to be scaled tohigher levels of integration than may be achieved with conventional DRAMcircuitry.

The devices discussed above may be incorporated into electronic systems.Such electronic systems may be used in, for example, memory modules,device drivers, power modules, communication modems, processor modules,and application-specific modules, and may include multilayer, multichipmodules. The electronic systems may be any of a broad range of systems,such as, for example, clocks, televisions, cell phones, personalcomputers, automobiles, industrial control systems, aircraft, etc.

The particular orientation of the various embodiments in the drawings isfor illustrative purposes only, and the embodiments may be rotatedrelative to the shown orientations in some applications. The descriptionprovided herein, and the claims that follow, pertain to any structuresthat have the described relationships between various features,regardless of whether the structures are in the particular orientationof the drawings, or are rotated relative to such orientation.

The cross-sectional views of the accompanying illustrations only showfeatures within the planes of the cross-sections, and do not showmaterials behind the planes of the cross-sections in order to simplifythe drawings.

When a structure is referred to above as being “on” or “against” anotherstructure, it can be directly on the other structure or interveningstructures may also be present. In contrast, when a structure isreferred to as being “directly on” or “directly against” anotherstructure, there are no intervening structures present. When a structureis referred to as being “connected” or “coupled” to another structure,it can be directly connected or coupled to the other structure, orintervening structures may be present. In contrast, when a structure isreferred to as being “directly connected” or “directly coupled” toanother structure, there are no intervening structures present.

In some embodiments, a semiconductor construction includes asemiconductor base and a gate extending into the base. A first region ofthe base adjacent the gate is a conductively-doped source region, and asecond region of the base adjacent the gate and spaced from the firstregion is a conductively-doped drain region. A gate dielectric comprisesa first segment between the source region and the gate, a second segmentbetween the drain region and the gate, and a third segment between thefirst and second segments. At least a portion of the gate dielectriccomprises ferroelectric material.

In some embodiments, a transistor comprises a gate, a source region, adrain region, and a channel region between the source and drain regions.The transistor also comprises a gate dielectric between the gate and thesource, drain and channel regions. The gate dielectric comprisesferroelectric material between the source region and the gate.

In some embodiments, a semiconductor construction comprises asemiconductor base and a gate extending into the base. A region of thebase on one side of the gate is a conductively-doped source region, anda region of the base on an opposing side of the gate relative to saidone side is a conductively-doped drain region. The drain region is moreheavily doped than the source region. The construction includes gatedielectric which comprises a first segment between the source region andthe gate, a second segment between the drain region and the gate, and athird segment between the first and second segments. The gatedielectric, along a cross-section, is configured as an upwardly-openingcontainer having the gate therein. The first segment of the gatedielectric comprises a first substantially vertical leg of thecontainer. The second segment of the gate dielectric comprises a secondsubstantially vertical leg of the container. The third segment of thegate dielectric comprises a bottom of the container. The gate dielectriccomprises non-ferroelectric material directly against ferroelectricmaterial, with the non-ferroelectric material being a boundary of thecontainer directly against the semiconductor base. The non-ferroelectricmaterial is thicker along the bottom of the container than along thefirst and second substantially vertical legs of the container.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

We claim:
 1. A semiconductor construction, comprising: a dielectricregion configured as an upwardly-opening structure; a gate extendinginto the upwardly-opening structure; a source region being adjacent oneside of the gate and a drain region being adjacent another side of thegate; the source region being spaced from the gate by a first segment ofthe dielectric region, and the drain region being spaced from the gateby a second segment of the dielectric region; a third segment of thedielectric region being a bottom of the upwardly-opening structure andbeing between the first and second segments; wherein a portion of thedielectric region comprises a non-ferroelectric material as an outerboundary, and comprises a ferroelectric material between thenon-ferroelectric material and the gate; and wherein the ferroelectricmaterial is only within the first segment.
 2. The semiconductorconstruction of claim 1 wherein the dielectric region is thicker alongthe third segment than along the first and second segments.
 3. Thesemiconductor construction of claim 1 wherein the non-ferroelectricmaterial comprises one or both of silicon dioxide and silicon nitride.4. The semiconductor construction of claim 1 further comprising acapacitor coupled to the drain region.
 5. The semiconductor constructionof claim 1 wherein the ferroelectric material comprises one or more ofHf, Zr, Si, O, Y, Ba, Mg and Ti.
 6. The semiconductor construction ofclaim 1 wherein the non-ferroelectric material comprises a thicknesswithin the first and second segments within a range of from about 10angstroms to about 20 angstroms, and comprises a thickness within thethird segment within a range of from about 25 angstroms to about 50angstroms.
 7. A transistor, comprising: a first dielectric regionbetween a gate and a source region; a second dielectric region betweenthe gate and a drain region; the first dielectric region comprising twomaterials; one of the two materials being ferroelectric material andanother of the two materials being a first non-ferroelectric material;the second dielectric region comprising only a single material, saidsingle material being a second non-ferroelectric material; and whereinthe first and second non-ferroelectric materials are differentcompositions relative to one another.
 8. The transistor of claim 7wherein the ferroelectric material comprises one or more of Hf, Zr, Si,O, Y, Ba, Mg and Ti.
 9. The transistor of claim 7 wherein the source anddrain regions are conductively-doped regions of a semiconductormaterial.
 10. The transistor of claim 9 wherein the source regioncomprises a dopant gradient.
 11. A transistor, comprising: a firstdielectric region between a gate and a source region; a seconddielectric region between the gate and a drain region; the firstdielectric region comprising two materials; one of the two materialsbeing ferroelectric material and another of the two materials being afirst non-ferroelectric material; the second dielectric regioncomprising only a single material, said single material being a secondnon-ferroelectric material; wherein the source and drain regions areconductively-doped regions of a semiconductor material; and wherein anentirety of the drain region is more heavily doped than any of thesource region.